High impedance electrode tip

ABSTRACT

An implantable lead, being either a fixed or retractable/extendable lead, having a distal tip electrode is adapted for implantation on or about the heart and for connection to a system for monitoring or stimulating cardiac activity. The electrode includes a mechanical fastener such as a fixation helix for securing the electrode to cardiac tissue, which may or may not be electrically active. The implantable electrode with a helical tip includes an electrode which has a distal end and a proximal end. A helix is disposed within the electrode, where the helix is aligned along a radial axis of the electrode. The electrode further includes one or more of the following features: the helix having a coating of an insulating material on a surface of the helix, a porous conductive surface at a base of the helix, or a porous conductive element at the end of the electrode having an insulating coating covering from 5-95% of the surface of the porous conductive element. The electrode may further include an electrode tip having a porous electrical conductive element, such as a mesh screen, disposed on a surface at the distal end of the electrode tip, which can be used as a sensing or pacing interface with the cardiac tissue.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This patent application is a division of U.S. patent applicationSer. No. 10/288,155, filed on Nov. 5, 2002, which is a division of U.S.patent application Ser. No. 09/121,288, filed on Jul. 22, 1998, nowissued as U.S. Pat. No. 6,501,994, which is a continuation-in-part ofU.S. patent application Ser. No. 08/998,174, filed on Dec. 24, 1997,entitled “RETRACTABLE LEAD WITH MESH SCREEN”, now abandoned, thespecifications of which are incorporated by reference herein.

FIELD OF THE INVENTION

[0002] The present invention relates generally to leads for conductingelectrical signals to and from the heart. More particularly, it pertainsto electrode tips for delivering electrical charges to the heart, and totips which tend to reduce power consumption from cells without reducingthe effective level of each pace.

BACKGROUND OF THE INVENTION

[0003] Leads implanted in the body for electrical cardioversion orpacing of the heart are generally known in the art. In particular,electrically transmissive leads may be implanted in or about the heartto reverse (i.e., defibrillate or cardiovert) certain life threateningarrhythmias or to stimulate contraction (pacing) of the heart.Electrical energy is applied to the heart via the leads to return theheart to normal rhythm. Leads have also been used to sense conditions,materials or events (generally referred to as “sense” or “sensing”) inthe body, such as in the atrium or ventricle of the heart and to deliverpacing pulses to the atrium or ventricle. Tachy leads generally can atleast sense, pace, and deliver defibrillation shocks. Brady leads can atleast perform the combination functions of pacing and sensing the heart.One of the available functions of the pacemaker or the automaticimplantable cardioverter defibrillator (AICD) is to receive signals froma lead and interpret signals. In response to these signals, thepacemaker can decide to pace or not pace. The AICD can decide to pace ornot pace, and shock or not shock. In response to a sensed bradycardia ortachycardia condition, a pulse generator produces pacing ordefibrillation pulses to correct the condition. The same lead used tosense the condition is sometimes also used in the process of deliveringa corrective pulse or signal from the pulse generator of the pacemaker.

[0004] Sick sinus syndrome and symptomatic AV (atrial-ventricular) blockconstitute two of the major reasons for insertion of cardiac pacemakerstoday. Cardiac pacing may be performed by the transvenous method or byleads implanted directly onto the ventricular epicardium. Most commonly,permanent transvenous pacing is performed using a lead positioned withinone or more chambers of the heart. A lead, sometimes referred to as acatheter, may be positioned in the right ventricle or in the rightatrium through a subclavian vein or other vascular port, and leadterminal pins are attached to a pacemaker which is implantedsubcutaneously. The lead may also be positioned in both chambers,depending on the lead, as when a lead passes through the atrium to theventricle. Sense electrodes may be positioned within the atrium or theventricle of the heart as appropriate for the particular condition orthe choice of the medical practitioner.

[0005] Pacemaker leads represent the electrical link between the pulsegenerator and the heart tissue which is to be excited. These pacemakerleads include single or multiconductor coils of insulated wire having aninsulating sheath. The coils provide a cylindrical envelope or tube,many times referred to as a lumen, which provides a space into which astiffening stylet can be inserted. The conductive coil is connected toan electrode in an electrode assembly at a distal end of a pacing lead.Typically, a terminal member is molded within a flexure sleeve at theproximal end of the pacing lead and connected to the proximal end of theconductive coil.

[0006] After the electrode assembly is positioned at a desired locationwithin the heart, it is desirable to provide some method for securingthe electrode assembly at that location. Mechanical fixation devices areused to firmly anchor the electrodes in the heart. One type ofmechanical fixation device used is a corkscrew, or a helix electrodeconnector. During placement of the lead, the tip of the lead travelsintravenously through veins and the heart. While traveling through theveins, the helix electrode connector at the tip of the lead may snag orattach to the side wall of the vein. Since this is highly undesirable asit may cause damage or other complications to a patient, retractablehelixes are one of the optional constructions which have been providedfor leads. In addition, temporary caps over the helix (such as anaqueous soluble cap, particularly a water soluble, innocuous organicmaterial such as a sugar, starch or other biologically inert, ordigestible material such as sugars, starches and the like (e.g.,mannitol, sorbitol)) may be formed over the helix or tip. Preferablythese materials are at least soluble or dispersible and preferably areinert or even digestible.

[0007] When using a retractable helix, the helix is extended and screwedinto the heart muscle by applying a torque to the other end of theconductor without use of any further auxiliary device or with a specialfixation stylet. A fixed or non-retractable helix electrode connectorneeds only to be positioned and secured to the heart muscle by theapplication of torque. If a soluble/dispersible cap is present on thehelix, the cap must be given sufficient time to dissolve or dispersebefore complete securement of the helix electrode connector isattempted. A lead must be capable of being firmly secured into the wallof the cardiac tissue to prevent dislodgement therefrom, while avoidingperforation of the electrode completely through the cardiac tissue.

[0008] The pulse generator circuitry and power supply work in concertwith the electrodes as a system which provides electrical pulses to theheart tissue. A low impedance electrode design may increase powerdelivery to the heart tissue, but at the same time, this higher energyusage results in shorter battery life. Shorter battery life isundesirable, since it increases the average number of surgicalprocedures to perform battery replacement for a patient.

[0009] There is a need for a body-implantable lead that has a helix forfixation to the wall of the atrium or ventricle of the heart. A separatedesirable feature in body-implantable leads is for a lead having anelectrode for positioning within the atrium or ventricle that allows fortissue in growth. Tissue in growth further enhances the electricalperformance of the lead. The lead and electrode are further stabilizedwithin the heart as a result of tissue in growth. Furthermore, there isa need for a relatively high pacing impedance electrode design whichoffers reasonable average voltage threshold with sufficient signalamplitude so that the pacing function would be effectively provided withreduced energy utilization and consequently extend battery life.

SUMMARY OF THE INVENTION

[0010] According to the present invention, there is provided abody-implantable lead assembly comprising a lead, one end of the leadbeing adapted to be connected to electrical supply for providing orreceiving electrical pulses. The other end of the lead comprises adistal tip which is adapted to be connected to tissue of a living body.The lead is characterized by having either a) a porous electrode at thebase of the helix and/or b) an insulating coating over a portion of thehelix so that the impedance is increased for the helix as compared to ahelix of the same size and materials without an insulating coating. Thelead also has an increased impedance or high impedance which can act toextend the life of the battery. The high or at least the increasedimpedance may be effected in any of a number of ways, including, but notlimited to one or more of the following structures: 1) a fully insulatedtissue-engaging tip with an electrode at the base of the insulated tip,2) a partially insulated engaging tip (only a portion of the surfacearea of the engaging tip being insulated), 3) a mesh or screen ofmaterial at the distal end of the lead, at the base of an extendedengaging tip (whether a fixed or retractable tip), 4) the selection ofmaterials in the composition of the mesh and/or tip which provide higherimpedance, 5) the partial insulative coating of a mesh or screen toincrease its pacing impedance, and 6) combinations of any of thesefeatures. There may be various constructions to effect the highimpedance, including the use of helical tips with smaller surface areas(e.g., somewhat shorter or thinner tips). There may also be a sheath ofmaterial inert to body materials and fluids and at least one conductorextending through the lead body. The use of these various constructionsin the tip also allows for providing the discharge from the tip in amore highly resolved location or area in the tip.

[0011] According to the present invention, there is provided abody-implantable lead assembly comprising a lead, one end being adaptedto be connected to electrical supply for providing or receivingelectrical pulses. The lead further comprises a distal tip which isadapted to be connected to tissue of a living body. The lead also has ahigh impedance to extend the life of the battery. There may be variousconstructions to effect the high impedance. There may also be a sheathof material at the distal end of the lead assembly, with the sheathbeing inert to body materials and fluids and at least one conductorextending through the lead body.

[0012] The distal tip electrode is adapted, for example, forimplantation proximate to the heart while connected with a system formonitoring or stimulating cardiac activity. The distal tip electrodeincludes an electrode tip (preferably with only a percentage of itsentire surface area being electrically conductively exposed—only aportion of the surface is insulated—to increase its impedance),preferably a mesh screen disposed at a distal end of the electrode tip,a fixation helix disposed within the electrode tip, and a helix guidingmechanism. The mesh screen preferably is electrically active, and thearea of the mesh screen and the percentage of electrically exposedsurface area of the electrode tip can be changed to control electricalproperties. Further, the mesh screen can entirely cover an end surfaceof the electrode tip, or a portion of the end surface in the form of anannular ring. In one embodiment, the helix guiding mechanism includes ahole punctured within the mesh screen. Alternatively, the helix guidingmechanism can include a guiding bar disposed transverse to a radial axisof the electrode. The helix is retractable, and is in contact with amovement mechanism. The movement mechanism provides for retracting thehelix, such as during travel of the electrode tip through veins. Thehelix is aligned with the radial axis of the electrode and travelsthrough the guiding mechanism. The mesh may be tightly woven orconstructed so that there are effectively no openings, or the mesh canbe controlled to provide controlled porosity, or controlled flow throughthe mesh.

[0013] In another embodiment, the electrode tip includes a mesh screenforming a protuberance on the end surface of the electrode tip. Theprotuberance is axially aligned with the radial axis of the electrode.The helix travels around the protuberance as it passes through the meshwhile traveling to attach to tissue within the heart. The helix alsotravels around the protuberance as it is retracted away from the tissuewithin the heart. If the mesh screen is insulated around theprotuberance, then a high impedance tip is created. Advantageously, theprotuberance allows for better attachment to the cardiac tissue withouthaving the electrode tip penetrating therethrough.

[0014] Additionally, a distal tip electrode is provided including anelectrode tip, a mesh screen disposed at a distal end of the electrodetip, a fixation helix disposed within the electrode tip, and a helixguiding mechanism. The electrode tip further may include a piston formoving the helix. The piston further may include a slot for receiving abladed or fixation stylet. When engaged and rotated, the piston providesmovement to the helix. The base provides a mechanical stop for the helixand piston when retracted back into the electrode tip.

[0015] In another embodiment, the distal tip assembly is adapted forimplantation proximate to the heart while connected with a system formonitoring or stimulating cardiac activity. A fixation helix/pistonassembly is housed by an electrode collar, housing, and base assembly.Attached to the proximal end of the helix is a piston which includes aproximal slot for receiving a bladed or fixation stylet. When a styletis engaged in the slot and rotated, the piston provides movement to thehelix. Depending on the embodiment, the fixation helix/piston assemblymay be electrically active or inactive. The electrode collar, housing,and base all house the fixation helix/piston assembly. The proximal endof the electrode collar is attached to the distal end of the housing.Furthermore, the proximal end of the housing is attached to the distalend of the base, and the proximal end of the base is directly attachedto the conductor coils of the lead.

[0016] A mesh screen may be attached to the distal tip of the electrodecollar. The mesh screen, in another embodiment, is electrically activeand serves as the electrode on the distal tip assembly. The tip may thenbe fully insulated to increase the impedance of the tip or may bepartially insulated (with preselected areas of the helix tip beinginsulated and other areas being non-insulated) to adjust the impedanceof the tip to the specific or optimal levels desired. The area of themesh screen can be modified to cover differing portions of the endsurface of the distal tip assembly to control electrical properties ofthe lead. The fixation helix travels through a guiding mechanism, wherethe guiding mechanism allows the fixation helix to be extended andretracted. In one embodiment, the helix guiding mechanism includes ahole formed within the mesh screen. Alternatively, the helix guidingmechanism can include a guiding bar disposed transverse to a radial axisof the electrode collar. The mesh screen and/or guiding bar also serveas a full extension stop when the helix is fully extended. The baseserves as a stop when the fixation helix/piston assembly is fullyretracted.

[0017] The provided electrode tip supplies a retractable helix and amesh screen which advantageously allows for sufficient tissue in-growth.The guide mechanism provides a convenient way to direct the rotation ofthe helix. A further advantage of the electrode tip is the providedmechanical stop. The mechanical stop aids in preventing over-retractionof the helix during the installation or removal of the electrode tip.

[0018] In yet another embodiment, the electrode uses a partiallyinsulated fixation helix to provide a relatively high pacing impedanceelectrode. The fixation helix is insulated using insulating coatingsover a portion of the fixation helix.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a first side elevational view illustrating a leadconstructed in accordance with one embodiment of the present invention.

[0020]FIG. 2A is a cross-sectional view of an electrode tip of a leadfor monitoring and stimulating the heart constructed in accordance withone embodiment of the present invention.

[0021]FIG. 2B is an end view of the electrode tip of the lead shown inFIG. 2A.

[0022]FIG. 3A is a cross-sectional view of an electrode tip of a leadfor monitoring and stimulating the heart constructed in accordance withone embodiment of the present invention.

[0023]FIG. 3B is an end view of the electrode tip of the lead shown inFIG. 3A.

[0024]FIG. 4A is a cross-sectional view of an electrode tip of a leadfor monitoring and stimulating the heart constructed in accordance withone embodiment of the present invention

[0025]FIG. 4B is an end view of the electrode tip of the lead shown inFIG. 4A.

[0026]FIG. 5A is a cross-sectional view of an electrode tip of a leadfor monitoring and stimulating the heart constructed in accordance withone embodiment of the present invention

[0027]FIG. 5B is an end view of the electrode tip of the lead shown inFIG. 5A.

[0028]FIG. 6 shows a partially insulated helical tip according to thepresent invention which increases the impedance of the tip as comparedto a fully non-insulated helical tip.

DETAILED DESCRIPTION OF THE INVENTION

[0029] In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which specific aspects ofthe broader invention may be practiced. These embodiments are describedin sufficient detail to enable those skilled in the art to practice boththe broad concepts of the invention as well as more limiting specificconstructions, and it is to be understood that other embodiments may beutilized and that structural changes may be made without departing fromthe spirit and scope of the present invention as disclosed herein.Therefore, the following detailed description is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

[0030] As noted previously, there are a number of ways in whichincreased impedance may be effected for mechanically fastened electrodeconnections in atrial/ventricular implantable catheters (AVIC) systems.These include at least the following: 1) a fully insulated tissueengaging tip (at least with respect to all surfaces that are inelectrical contact or electrically active physical relationship to heartmuscles so that a pace would be effective if discharged at that portionof the tip), 2) a partially insulated engaging tip (only a portion ofthe surface area of the engaging tip being insulated, preferably thereis sufficient coating so that there is at least 5%, or at least 10%, orat least 20 or 30%, or at least 40, 50 or 60%, or at least 70, 75, 80 or90% of the surface area of the tip which can discharge to heart muscle[or as percentages of the entire tip or as percentages of the entire tipthat extends physically beyond the end plane of the catheter and whichmay therefore penetrate tissue or muscle]), 3) a porous, electricallyconductive element, such as a mesh or screen of material at the proximalend of the helix or the distal end of the lead (excluding the helix), atthe base of an extended engaging tip, 4) the selection of materials inthe composition of the mesh and/or tip which provide higher impedance,5) the partial insulative coating of a porous conductive element, suchas the mesh or screen to increase its impedance, and 6) combinations ofany of these features. There may be various constructions to effect theincreased or high impedance, including the use of helical tips withsmaller surface areas (e.g., somewhat shorter or thinner tips). Theremay also be other elements associated with the catheter and/or leads,such as a sheath of material inert to body materials and fluids,circuitry, microcatheters, and at least one conductor extending throughthe lead body.

[0031] One aspect of the present invention comprises an implantableelectrode with a helical tip comprising:

[0032] an electrode having a distal end and a proximal end; and

[0033] a helix disposed within the electrode, which helix is alignedalong a radial axis of the electrode towards the distal end, and whichhelix is either retractable or fixed; and

[0034] the implantable electrode having at least one feature selectedfrom the group consisting of:

[0035] a) the helix having a coating of an insulating material on itssurface which covers at least 5% of its surface area but less than 95%of its surface area (which is exposed beyond the distal end of theelectrode),

[0036] b) the helix extending beyond the distal end of the electrode andthe distal end of the electrode having a porous conductive surface at abase of the helix,

[0037] c) a porous conductive element such as a screen or mesh at a baseof the helix, which is retractable/extendable, with the helix beingeither active or inactive (electrically), and

[0038] d) a partially insulated (partially insulation coated) porousconductive element (e.g., screen or mesh) at the base of an active orinactive, retractable/extendable or fixed helix.

[0039] The implantable electrode preferably has the helix with a coatingof insulating material on its surface which covers from 5-100% (to 100%where there is an additional electrode element within the system) or5-95% of surface area of the helix beyond the distal end of theelectrode. Alternatively, the surface of the helix is that which isconsidered to be in electrically discharge-functional physicalrelationship with tissue or muscle into which it is embedded. Forpurposes of measuring or determining the distal end of the electrode,the tip extends beyond a tubular or cylindrical housing or structuralportion which is considered the electrode, and the tip is an engagingportion that extends beyond the housing portion of the electrode. Thedistal end of the electrode is usually characterized as the end of thecylindrical housing or tubing carrying the tip, circuits, conductiveelements, guides, etc. It is more preferred that the helix of theimplantable electrode has a coating of insulating material on it surfacewhich covers from 5-95% or 10-90% of the surface area of said helixbeyond the distal end of the electrode.

[0040] A lead 10 is illustrated in FIG. 1. The lead 10 comprises a leadbody 11, an elongate conductor 13 contained within the lead body, and alead tip 20 with an optional retractable tip assembly 24 contained inthe lead tip 20. In addition, a stylet 14 is shown inserted into thelead body 11. A helix 100 (FIGS. 2A-5A), which consists of an electricalconductor coil, is contained in the retractable lead tip 24. In analternative practice of the invention, the helix 100 extends andretracts by rotation of the stylet 14, as will be discussed furtherbelow. A Brady lead body is shown, although the invention could beincorporated with other leads, such as Tachy leads. The lead body 11consists of electrical conductors 13 which are covered by abiocompatible insulating material 22. Polymers, such as silicone rubber,fluorinated resins, polyacrylates, polyamides ceramic or compositematerials or other insulating material can be used for covering the leadbody 11.

[0041] In one embodiment shown in FIGS. 3 and 3A, the helix 100 isformed of electrically conductive material offering low electricalresistance and also resistant to corrosion by body fluids. Abiocompatible metal, such as titanium or platinum-iridium alloy is anexample of a suitable material. Alternatively, the helix 100 iselectrically inactive or insulated. In one embodiment, the helix 100 maybe coated with an insulative material (not shown) or may be constructedof a rigid, corrosion resistant, non-electrically-conductive material(e.g., a ceramic). A housing 182, described in further detail below, ismade from an electrically conductive material and covered with aninsulating material such as a synthetic or natural polymer such as asilicone rubber. The housing 182 is directly connected to an electricalconductor within the lead 120. These materials are additionally suitablebecause they tend to be biologically inert and well tolerated by bodytissue.

[0042] The helix 100 defines a lumen and thereby is adapted to receive astiffening stylet 14 that extends through the length of the lead. Thestylet 14 stiffens the lead 120, and can be manipulated to introduce anappropriate curvature to the lead, facilitating the insertion of thelead into and through a vein and through an intracardiac valve toadvance the distal end of the lead 120 into the right ventricle of theheart (not shown). A stylet knob 154 is coupled with the stylet 14 forrotating the stylet 14 and advancing the helix 100 into tissue of theheart.

[0043] In one embodiment, as shown in FIGS. 2A and 2B, a lead 310 has anelectrode tip 320 which is provided with a mesh screen 330. The meshscreen 330 completely encapsulates the diameter of the lead, and mayserve, at least in part, as a pacing/sensing interface with cardiactissue. If the helix 100 is electrically active, it too can help serveas a portion of a pacing or sensing interface. The mesh screen 330 is ofa porous construction, preferably made of electrically conductive,corrosion resistant material. Using a mesh screen 330 having a porousconstruction allows for fibrotic ingrowth. This provides for a furtheranchoring of the lead tip 320 and also increases the sensing capabilityof the lead 310 by increasing the surface area in contact with thecardial tissue. The mesh screen 330 may be attached to an electrodecollar 40, which is electrically active. In a retractable cathetersystem, a housing 380, which is electrically conductive, encapsulatesthe piston 350 and the fixation helix 100. Insulation 382 is disposedabout the housing 380 and collar 40.

[0044] Disposed within the lead 310 is a lead fastener 100 for securingthe lead 310 to cardiac tissue. The lead fastener 100 can be disposedalong the radial axis 15 of the electrode lead 310. In this embodiment,the lead fastener comprises a fixation helix 100. The fixation helix 100can be made electrically active or inactive as discussed above. Attachedto the fixation helix 100 in a retractable tip system is a piston 350.The piston 350 is configured to mate with a bladed locking stylet 14 ata stylet slot 354, and acts as an interface between the stylet 14 andthe helix 100. The stylet 14, coupled with the piston 350 at the styletslot 354, extends and retracts the fixation helix 100 when the stylet 14is rotated. The piston 350 can either be electrically active orinactive. The piston 350 also has a slot 352, which allows the piston350 to mate with a base 360.

[0045] Fitted with a knob 362, as shown in FIG. 2A, the base 360 mateswith the slot 352 of the piston 350. The base 360 serves as a stop oncethe fixation helix 100 is fully retracted. The electrically conductivebase 360 also allows passage of a bladed locking stylet 14 andattachment of electrode coils (not shown).

[0046] In addition, the lead 310 has a guide groove 370. The groove 370is formed by puncturing a hole (not shown) within the mesh screen 330,although the guide groove 370 can be formed by other methods known bythose skilled in the art. Having a circular cross-section, the guidegroove 370 may have a diameter greater than that of the conductorforming the helix 100. The groove 370 is disposed within the mesh screen330, and directs the fixation helix 100 from its retracted position, asillustrated in FIG. 2A, to an extended position (not shown). The groove370 also reversibly directs the fixation helix 100 from an extendedposition to the retraction position.

[0047] In a second embodiment, as shown in FIGS. 3A and 3B, a lead 110has an electrode tip 120 which is provided with a mesh screen 130. Themesh screen 130 completely encapsulates the diameter of the lead orelectrode tip 120, and serves as the pacing/sensing interface withcardiac tissue. The screen 130 is of a porous construction, made ofelectrically conductive, corrosion resistant material. Using a meshscreen 130 having a porous construction allows for fibrotic ingrowth.This provides for a further anchoring of the lead tip 120 to tissue andalso increases the sensing capability of the lead 110. The sensingcapability is enhanced because the mesh screen 130 has more surface areathan corresponding solid material. The ingrowth of fibrotic tissue intothe mesh screen 130 increase the sensing capability of the lead 110 byincreasing the surface area in contact with the cardiac tissue.Furthermore, the geometry of the mesh screen 130, particularly anyprotuberance, as will be discussed below, creates a high pacingimpedance tip.

[0048] The mesh screen 130 may form a protuberance 135 from a flat edgeportion 137 of the mesh screen 130 in a generally central portion of theelectrode tip 120. The protuberance 135 may be generally circular incross-section, but may be any shape (e.g., truncated cylindrical,truncated pyramidal, oval, ellipsoidal, etc.) as a result of design orcircumstance which provides a flat or conformable surface (preferablynot a rigid, sharp face which will not conform to tissue) abuttingtissue, and preferably has a diameter smaller than a diameter of thelead 110. In addition, the protuberance 135 is aligned with the radialaxis 15 of the lead 110. Sintered to an electrode collar 40, a processknown by those skilled in the art, the mesh screen 130 is attached tothe electrode tip 120. The electrode collar 40 is electrically active.

[0049] Disposed within the electrode lead 110 is a lead fastener forsecuring the electrode lead 110 to cardiac tissue. The lead fastener canbe disposed along the radial axis 15 of the electrode lead 110. In thisembodiment, the lead fastener comprises a fixation helix 100. Thefixation helix 100 can be made electrically active or inactive to changesensing and pacing characteristics, as discussed above. Attached to thefixation helix 100 is a piston 150. The piston 150 is configured to matewith a bladed locking stylet 14, thereby providing a movement assembly.The stylet 14 extends and retracts the fixation helix 100 when thestylet 14 is rotated. The piston 150 can either be electrically activeor inactive. The piston 150 also has a slot 152. The slot 152 of thepiston 150 allows the piston 150 to mate with a base 160 upon fullretraction.

[0050] The base 160 is modified with a knob 162 to mate with the slot152 of the piston 150. The knob 162 mates with the piston 150 to preventover-retraction once the helix 100 has been fully retracted. The stylet14 operates to advance the fixation helix 100. As the implanter rotatesthe stylet 14, the stylet 14 engages the piston 150 at the stylet slot154 and rotates the piston 150, which moves the fixation helix 100through a guide groove 170. The guide groove 170 is for ensuring thatthe fixation helix 100 is properly guided out of and into the end of theelectrode. Once the fixation helix 100 is fully retracted, the base 160serves as a mechanical stop. The base 160 also allows passage of abladed locking stylet 14 and attachment of electrode coils.Additionally, the base 60 is electrically active.

[0051] The electrode lead 110 also has a guide groove 170. The groove170 is formed by puncturing a hole within the mesh screen. Having acircular cross-section, the groove 170 has a diameter greater than thatof the conductor forming the helix 100. The groove 170 is disposedwithin the mesh screen 130, and directs the fixation helix 100 from itsretracted position, as illustrated in FIG. 2A, to an extended position(not shown). During implantation, after the electrode is in contact withtissue at the desired location in the heart, the stylet 14 is rotatedwhich causes the piston to advance the fixation helix out of the groove170. As the fixation helix 100 is placed in an extended position, thehelix 100 travels through groove 170 and circles around the protuberance135. The groove 170 also directs the fixation helix 100 from an extendedposition to the retracted position. Advantageously, the mesh screen 130prevents the implanter from overextension and advancing the helix 100too far into the tissue. An electrically conductive housing 180encapsulates both the piston 50 and the fixation helix 100. Insulation182 covers the housing 180, the collar 40, and a portion of the meshscreen 130. The insulation 182 over the mesh screen 130 controls theimpedance of the electrode tip 120.

[0052] In a third embodiment as shown in FIGS. 4A and 4B, a lead 10 hasan electrode tip 20 which is provided with a mesh screen 30. The meshscreen 30 completely encapsulates the diameter of the lead tip. Sinteredto an electrode collar 40, the mesh screen 30 is attached to theelectrode tip 20. The electrode collar 40 is electrically active. Ahousing 80 is disposed about the helix 100, and is electrically active.Insulation 82, encompasses the housing 80 and collar 40.

[0053] Disposed within the lead 10 is a lead fastener for securing thelead 10 to cardiac tissue. The lead fastener can be disposed along theradial axis 15 of the lead 10. In this embodiment, the lead fastenercomprises a fixation helix 100. The fixation helix 100 can be madeelectrically active or inactive to change sensing and pacingcharacteristics.

[0054] The helix 100 is of a well known construction. Using a conductorcoil such as helix 100 has been shown to be capable of withstandingconstant, rapidly repeated flexing over a period of time which can bemeasured in years. The helix 100 is wound relatively tightly, with aslight space between adjacent turns. This closely coiled constructionprovides a maximum number of conductor turns per unit length, therebyproviding optimum strain distribution. The spirally coiled springconstruction of helix 100 also permits a substantial degree ofelongation, within the elastic limits of the material, as well asdistribution along the conductor of flexing stresses which otherwisemight be concentrated at a particular point.

[0055] Attached to the fixation helix 100 is a piston 50. The piston 50is configured to mate with a bladed locking stylet 14. The piston 50advances the fixation helix 100 once the lead is placed in positionwithin the heart. The piston 50 can either be electrically active orinactive. The piston 50 also has a slot 52 and a stylet slot 54. Thestylet 14 couples with the stylet slot 54 and extends or retracts thefixation helix 100 when the stylet 14 is rotated. The slot 52 of thepiston 50 allows the piston 50 to mate with a base 60 when the helix 100is retracted to prevent over retraction. The base 60 is configured witha knob 62 to mate with the slot 52 of the piston 50. Once the fixationhelix 100 is fully retracted, the base 60 serves as a stop at fullretraction. The base 60 also allows passage of a bladed locking stylet14 and attachment of electrode coils. In addition, the base 60 iselectrically active.

[0056] The lead 10 also includes a guiding bar 70. Extending across thediameter of the tip, the guiding bar 70 is generally cylindrical inshape. The guiding bar 70 directs the fixation helix 100 from itsretracted position, as illustrated in FIG. 2A, to an extended position(not shown) as the piston 52 advances the helix 100. The guiding bar 70also directs the fixation helix 100 as it is retracted from an extendedposition to the retraction position through the mesh screen. Although aguiding bar 70 is described, other types of guiding mechanisms can beused such as helical passageways, threaded housings, springs, and areconsidered within the scope of the invention. Additionally, the lead 10is provided with a seal (not shown) for preventing entry of body fluidsand tissue from entering the lead through the opening therein. The sealcould be a puncture seal between the piston 50 and the base 60.Alternatively, O-rings could be used to seal the electrode.

[0057] In a fourth embodiment as shown in FIGS. 5A and 5B, a lead 210has an electrode tip 220 which is provided with a mesh screen 230. Themesh screen 230 forms an annular ring having an open center, where theannular ring is centered at a radial axis 15 of the electrode lead 210.The mesh screen 230 provides more surface area than a smooth tippedelectrode which aids in sensing. The removal of the center portion ofthe mesh screen creates a high impedance pacing tip due to the nature ofthe surface geometry. Sintered, fused, bonded, adhesively secured ormechanically attached to an electrode collar 40, the mesh screen 230 isattached to the electrode tip 220. The electrode collar 40 iselectrically active.

[0058] Disposed within the lead 210 is a lead fastener for securing thelead 210 to cardiac tissue. The lead fastener can be disposed along theradial axis 15 of the electrode lead 210. In this embodiment, the leadfastener comprises a fixation helix 100. The fixation helix 100 can bemade electrically active or inactive as discussed above. Attached to thefixation helix 100 is a piston 250. The piston 250 has a stylet slot 254and is configured to mate with a bladed locking stylet 14. The stylet14, coupled with the piston 250 at the stylet slot 254, extends andretracts the fixation helix 100 when the stylet 14 is rotated. Thepiston 250 can either be electrically active or inactive. The base 260serves as a stop once the fixation helix 100 is fully retracted. Thebase 260 also allows passage of a bladed locking stylet 14 andattachment of electrode coils. The base 60 is electrically active.

[0059] Additionally, the electrode lead also has a guiding bar 270. Theguiding bar 270 directs the fixation helix 100 from its retractedposition, as illustrated in FIGS. 5A and 5B, to an extended position(not shown). The guiding bar 270 also directs the fixation helix 100from an extended position to the retracted position. Although a guidingbar 270 has been described, other types of mechanisms could be used toextend the helix, and are considered within the scope of the invention.A housing 280 encapsulates the piston 250 and the fixation helix 100,and insulation 282 is disposed over the housing 280 and collar 40.

[0060] Insulation generally covers the housing, the collar, and aportion of the electrical discharge surface (e.g., the cathode, thehelix and/or the porous material or mesh). The insulation over the meshscreen further controls the impedance of the electrode tip. Theinsulated coating, whether present on the helix or the mesh or otherelements which are potentially electrically active or on whichelectrical activity is to be suppressed, should be biocompatible,non-thrombogenic, and otherwise safe for implantation. The insulationcoating should be of dimensions which effect the insulation, increasethe impedance (where desired), but which dimensions do not interferewith the performance of the tip, the lead or the helix or the health ofthe patient. The insulation is present as a coating ( a material whichtends to conform to the surface rather than completely reconfigure it,as would a lump of material). The coating usually should be at least 0.5microns in thickness, usually between 0.5 and 100 microns, preferablybetween 1.0 and 30 or 50 microns, more preferably between 1 and 20microns, still more preferably between 1.5 and 15 microns, and mostpreferably between 1.5 or 2.0 microns and 10 or 15 microns. The coatingmay be provided by any convenient process, such as electrophoreticdeposition, dip coating, spin coating, in situ polymerization, vapordeposition, sputtering and the like. Any insulating material is useful,such as polymers, ceramics, glasses, and the like, but because of theirconvenience in application, flexibility and availability, polymers arepreferred. Polymers from such classes as polyesters, polyamides,polyurethanes, polyethers, polysiloxanes, polyfluorinated resins,polyolefins, polyvinyl polymers, polyacrylates (includingpolymethacrylates), and the like may be used with various leads and tipsaccording to the practice of the present invention. PARYLENE is apreferred material, as described herein, with a thickness of between 1.5and 10 microns.

[0061] In yet another embodiment, a partially insulated fixation helixis used to provide a relatively high impedance electrode design. Leadscomprising a distal or electrode end and a proximal or connector end maybe used. A “miniature” wire-in-basket porous electrode may be sinteredupon the distal end of a metallic pin, provided with a blind hole.Circumferential to this subassembly, a sharpened wire fixation helix maybe positioned and attached at a general location proximal to theelectrode by any convenient means which allows electrical continuity.This attachment includes, but is not limited to, crimping, spot welding,laser welding, the use of grooves upon the surface of the pin, the useof thin metallic overband (also not shown) or any combination thereof. Aportion of this fixation helix is provided with an extremely thin layerof a biostable, biocompatible polymer, which, inter alia, provideselectrical insulation between the fixation helix and the cardiac tissue.In one embodiment, the insulated portion is the majority of the fixationhelix, leaving a relatively small uninsulated region of fixation helix.This approach offers increased impedance to reduce energy dissipation inpulsing functions, such as pacing functions. Other varying embodimentsinclude, but are not limited to, a portion which is approximately orsubstantially equal to half of the fixation helix, and a portion whichis approximately or substantially equal to a minority of the fixationhelix. Such embodiments provide different amounts of uninsulated regionand different amounts of impedance. The thin coating of electricallyinsulating coating must usually be at least 1 micron in thickness toprovide a significant insulating effect, depending upon its insulatingability and properties. The thickness of the coating is limitedprimarily by physical limitations on the system. The coating can not beso thick as to interfere with the fastening ability of the helix or toincrease the size of the helix beyond that which is tolerable for theuse of the helix and the patient. Typically, the coating is at least onemicron up to about 100 microns, more typically the coating is between 1and 30 microns, preferably between 1.5 and 20 microns, more preferablybetween 1.5 and 15 microns, and most preferably between 2 and 10microns. The material used for the coating should, of course, bebiocompatible and even more preferably non-thrombogenic. Materials suchas PARYLENE™, polyurethanes, polyacrylates (includingpolymethacrylates), polyesters, polyamides, polyethers, polysiloxanes,polyepoxide resins and the like can be used. PARYLENE material includesa thermoplastic film polymer base upon para-xylylene. Crosslinkedpolymers within these classes may be preferred for their resistance tobreakdown and their physical durability. As the coating is to bemaintained within the body of a recipient, the coating compositionshould not be water-soluble or aqueous soluble within the parameters andenvironment encountered within animal bodies (e.g., it should not besoluble within blood, serum or other body fluids with which it mightcome into contact).

[0062] To the proximal end of this pin, a metallic conductor coil may beconveniently attached to provide electrical connection to theimplantable pacemaker (not shown) by means of a connector. In oneembodiment, local (e.g., steroid or other medicinal) therapy is providedby a (e.g., circumferential) steroid/polymer matrix positionedimmediately proximal to the porous electrode. In one embodiment, thecircumferential steroid/polymer matrix is provided with a distal taper.Other embodiments include other distal configurations, including, butnot limited to, non-tapered or “inflated” configurations. In oneembodiment, an internalized, medicinal or biologically active (e.g.,steroid) releasing matrix is used. Proximal to this biologically active(e.g., steroid) eluting matrix, a generally cylindrical polymeric tubing(this is the preferred shape, but the shape is a matter of choice) 820is used to provide electrical insulation of this entire assembly. In oneembodiment, the lead is “unipolar.” In one embodiment, an ablativeprotective covering positioned over the entirety of distal end is used(not shown). One example of such a covering is the mannitol “Sweet Tip”®electrode of Guidant Corporation's Cardiac Rhythm Management Group. Inone embodiment, a “bipolar” lead is provided with the distal electrodefeatures described.

[0063] During an in vitro evaluation of this electrode design, polymericcoatings intended to partially insulate the fixation helix were preparedand evaluated. In one embodiment, the PARYLENE coating is extremely thin(˜3μ), providing a coating with uniform coverage which is adherent tothe metallic substrate, and which is controllable to provide an abruptmargin. The silicone rubber coating is known to be somewhat thicker(˜10μ), uniform in coverage, somewhat less adherent to the metallicsubstrate, and controllable to an abrupt margin. Other coatings may beused without departing from the spirit and scope of the presentinvention.

[0064] The PARYLENE or other insulative coating effectively increases invitro “pacing impedance.” Application of a non-continuous or partiallyextensive coating of an electrically insulating polymer such as PARYLENEto the metallic fixation helix produces the desired increase inimpedance compared to an uninsulated helix as well as other existingdesigns. For example, it has been demonstrated that one embodiment usinga coated fixation helix provides a pacing impedance of overapproximately 800 ohms which is larger than the impedance of someelectrodes using an uncoated fixation helix. The post-implant pacingimpedance of an embodiment using a coated fixation helix remains higherthan that of typical electrodes using an uncoated fixation helix. In oneexperiment, a coated fixation helix using PARYLENE as an insulatinglayer provided over 1200 ohms average pacing impedance on the day ofimplantation and over 900 ohms ten days after the implant.

[0065] Additionally, post-implant average voltage threshold of thePARYLENE insulated miniaturized electrode is less than the other highimpedance electrodes. Such performance is considered to be desirable. Inone experiment, an embodiment with a coated fixation helix 802 having avoltage threshold of approximately 0.2 volts on the day of implant wasmeasured at about 0.7 volts at ten days after the implant (using a 0.5ms pulse width). An electrode with an uncoated fixation helixdemonstrated over 0.8 volts average voltage threshold at ten days afterthe implant, illustrating the benefits of the coated fixation helix.

[0066] An additional benefit is that the coated fixation helixembodiments may provide an improvement in both the implant as well aspost-implant average S-wave amplitude detection.

[0067] The miniaturized high impedance, positive fixation porouselectrode technology described here provides the following advantagesover the prior art. For one example, the coated fixation helixembodiments provide an electrode where the benefits of high impedancepacing are realized through downsizing the porous electrode andinsulating the fixation helix. Downsizing of the porous electrode may beaccomplished, for example, by having a smaller porous (e.g., mesh)electrode supported on a non-conductive surrounding support element(e.g., a polymeric or composite film with a mesh central area,particularly a mesh truncated conical or pyramidal area of flexible,conductive mesh). An area of the completely conductive mesh may also bediscontinuously coated leaving a conductive central or conductive raisedarea, particularly surrounding a contact, engaging element, or helix.Further, an external steroid collar provides a fabrication advantagesince such a component can be readily mass produced compared to smallercomponents with elaborate profiles. Still further, fabrication of a leadwith this external collar is streamlined. The higher impedance designconserves battery power to provide longer battery life with fewerbattery replacements. Other benefits exist which are not described indetail herein, however, which those skilled in the art will appreciate.

[0068]FIG. 6 shows a high impedance catheter tip 800 with a partiallyinsulated tip 802 and a partially insulated mesh 808. The partiallyinsulated tip (or helix) 802 extends from a base, proximal end 830 to adistal, pointed end 834 with a middle portion 836 lying between proximalend 830 and distal end 834. Helix 802 comprises one fully insulatedsection 804 which begins at distal, pointed end 834 and extends to, andends with, middle portion 836 and one uninsulated section 806 whichextends from the end of the fully insulated section within middleportion 836 to base, proximal end 830. The partially insulated mesh 808comprises a first area 810 of the mesh 808 which is insulated and secondare 812 of the mesh 808 which is not insulated. The impedance of thecatheter tip can be readily controlled by the amount of surface area ofthe helical tip itself and the area of the mesh (if present) which isinsulated. With a fixed conductivity in the tip and the mesh (ifpresent), the impedance can be increased by increasing the percentage ofthe surface area of the tip or mesh which is insulated.

[0069] A hole 820 is shown in the mesh 808. The mesh 808 may be flat andflush with the end 822 of the catheter 816 or may be partially wrapped(not shown) over the end 820 or inside the end 820 to affix the mesh tothe catheter 816. The mesh 808 may also be hemispherical, truncatedconical, truncated pyramidal or any other shape which may assist inallowing the mesh 808 to more compliantly contact tissue (not shown)surface to transmit the pacing signal or discharge. Within the catheter816 may be a soluble, elutable or dispersible material which carriesmedication or biologically active material along with the catheter. Forexample, anti-inflammatants, antibiotics, analgesics, pain-reducingmedication, vitamins, anti-viral medication, or the like may betransmitted to the attachment site along with the catheter by inclusionwithin a material 814 carried within or on the catheter 816.

[0070] The coating of insulation on the helical tip or mesh may beapplied by any convenient method, including, but not limited to coating(e.g., dip coating), printing, spraying, brush application, resistapplication and removal and the like. The insulation may also containactive ingredients (such as those recited within material 814) tobenefit the patient. The insulation carrying the active material mustnot be soluble, so a polymer or other material that is porous or haselutable materials must be used. The material delivery does not have tobe coextensive with the life of the implant or the tip, and delivery ofthe material may be desirable only over a short time period afterinsertion of the helical tip and catheter.

[0071] A soluble or dispersible protective cap may also be placed overthe helical tip to reduce the possibility of any incidental damage whilecatheterizing or moving the tip within a patient. As previously noted,the cap material should preferably be biocompatible or even digestibleand may include such materials as natural and synthetic materials suchas sugars, starches, gelation (unhardened), gums, resins, polymers, andthe like. All components of the catheter and tip which are exposed tothe tissue or fluids within a patient should be non-thrombogenic, andbio-acceptable. There are extensive classes of commercially availablematerials which meet these needs for metal, polymeric, composite andother materials described within the practice of the present invention.

[0072] It is to be understood that the above description is intended tobe illustrative, and not restrictive. Although the use of the lead hasbeen described for use in a cardiac pacing system, the lead could aswell be applied to other types of body stimulating systems. Many otherembodiments will be apparent to those of skill in the art upon reviewingthe above description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

What is claimed:
 1. A distal tip electrode adapted for implantation onor about the heart and for connection to a system for monitoring orstimulating cardiac activity, said electrode comprising: an electrodetip; a mesh screen disposed at a distal end of the electrode tip; asurface at the distal end of the electrode tip; a fixation devicedisposed within said electrode, said fixation device adapted for travelalong radial axis of the electrode through said surface; a guidingmechanism for directing movement of the fixation device during travel;and a movement assembly, said movement assembly for providing movementto said fixation device.
 2. The distal tip electrode as recited in claim1, wherein said fixation device comprises a helix.
 3. The distal tipelectrode as recited in claim 1, wherein said movement assemblycomprises a piston.
 4. The distal tip electrode as recited in claim 3,wherein the piston has a slot therein, and the base further comprises aknob, said slot for mating with said knob.
 5. The distal tip electrodeas recited in claim 3, wherein the slot is mated with said knob to forma stop mechanism for said fixation device.
 6. The distal tip electrodeas recited in claim 1, wherein the mesh screen has a groove guidedisposed therein.
 7. The distal tip electrode as recited in claim 3,wherein said distal tip electrode further comprises a seal, said sealdisposed between said piston and said base.